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Discussion Starter · #1 ·
Everytime I go fill up I notice a ton of pressure in the tank when I take the cap off. If im not mistakin', gas caps should be vented. If not, they would fail emmissions of todays cars for example.

At the bone yard I got a cap from a Celica today with the same results. Anybody tried other makes or models that is vented and doesnt spill on a full tank?
 

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I don't know. I am havin the same problem with my Toyota pickup. I am getting gas pressurizing and spilling out the charcoal canister. But it could be because I relocated the charcoal canister. Or it could be because I have barely any gas.
 

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i have the ven line left open and never noticed any problem .... aka no pressure ever!
I was reading that your fuel pump can wear out faster if you don't have the proper tank pressure. I think the proper pressure was 2 psi could be wrong.
forgot to say its been 6 years now and 1000rwhp
 

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Iv never heard about a situation like that before. But I would imagine that the gas pump works harder to keep gas flowing under pressure.


I want a Toyota pickup. Like the 85 SR5 one on back to the future.

I wanted that truck when I was 6 or 7 when that movie came out.
 

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The gas cap is vented, but it's got a check valve in it that only lets air IN the tank, not out. The check valve in the gas cap has a spring in it to hold it closed, so it takes a little bit of vacuum in the take before the check valve opens to let air in. The vacuum comes from the engine using up the fuel in the tank. This is the way out MKIII's allow air into the tank... other cars (especially newer cars) do it differently. This is why solid and fully vented gas caps even exist. They are not designed for use on our cars though.

The reason for the check valve stopping the air from going out is that any air that goes out will have fuel vapor in it. If the fuel vapor goes out, it pollutes the air. They call it "evaporative emissions".

The tank isn't supposed to have positive pressure either. The fuel sloshing around, or the fuel being heated up (by the pump, fuel rail, or just the car sitting in the sun) will make the fuel evaporate and build pressure in the tank. The pressure is designed to exit through the breather tube (or vent line, whatever name you prefer), which runs off the top of the tank, along the bottom of the car parallel with the fuel lines, and into the charcoal canister. The charcoal filters the fuel vapor out of the air, and the clean air is freely vented out of the other side of the charcoal canister.

While the car is running, under certain conditions, the ECU opens a "purge control" solenoid, which allows the engine to pull vacuum through the charcoal canister (on a separate line from the tank vent line) to reclaim the fuel vapor that collected in the canister. The tank can still freely vent while this is happening. Some older cars use an engine temp controlled "vapor VSV", in combination with a check valve to close the line under boost, instead of an electric solenoid. I think the 7M uses the older mechanical style valve, but I know my 1JZ has an electric purge solenoid.

I test these systems all the time at work... the most common code to get on an OBD2 car is a leak in the vapor control system... P0442 "small evap leak" (which is literally a pinhole sized leak, and is always a bitch to find).

So anyway... if your car is building excess negative pressure, try a new gas cap (the proper one way vented kind). If it's building positive pressure check for clogged vent lines or a clogged charcoal canister. I've seen everything from spiders nesting in the canister vent tube to a flood title car that had mold growing in the charcoal and clogging the canister. Rock/road debris damage and rust clogged vent tubes are also common.
 

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^^ never knew that...you learn something new everyday

I had the same concern in my mk1 mr2 but never thought it to be anything major. Ended up driving to nh and back (about 250 miles) and stopped to get gas 5 minutes from home. I opened the gas gap and got sprayed with a small amount of gas. Turned out someone syphoned gas at some point and just pushed the hose down. I'm sure it's not that, but just figured I would throw it out there.
 

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Good stuff Destrux. It surely is a commonly overlooked system. So I have my canister hooked up (toyota pickup 22RE) but when I parked my truck there was a bunch of fuel leaking onto the floor. I barely have any fuel in my tank but what could cause that issue? I am scared to fill my tank now because I might lose it all. I have to figure it out before I begin driving my truck again.
 

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Discussion Starter · #17 ·
destrux - Thats a whole lot that I had no idea about. Not that the car doesnt perform well but it was a concern. This summer I have a road trip ahead and just beginning to think that it may effect efficiency on the highway. Older 90's style Celica basicly has the same cap style. Same effect. Sometimes when I open the cap, it would either suck in or blow out alot of air after a longer period of driving. I cant really tell if its vacuum or pressure. Its hard to tell.

Would this disturb gas mileage or even put the fuel pump under more pressure?
 

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Nappa sells a pre-vented cap for our cars, it allows the pressure to escape before the cap comes off completely, so you don't get the crazy gas vapor rush.
 

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I just noticed this the past 2 times I filled up, never before since then. It happened today. My air gasp didn't last 8 seconds tho lol - maybe like 3 or 4 but still....?

I was well past E (by accident) today and was nowhere near a gas station lol. Can it really be just from being really low on gas?
 

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Destrux, Good information.

We have a smoke machine to check for evap leaks which is great and efficient way to find small pin hole leaks. It's worth it's money when diagnosing small vacuum leaks on an entire system quickly.
 
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